Links between spatial differences in phytoplankton composition and changes in environmental variables in the Vaal River, South Africa, were investigated. Multivariate analysis showed that the most important environmental variables influencing algal abundance were turbidity, conductivity, nutrient concentrations and temperature. Of these variables conductivity, phosphorus, nitrogen and SiO2-Si showed spatial differences in concentration. The average nutrient concentrations (phosphorus, nitrogen and SiO2-Si) showed downstream decreases, while the average conductivity increased in a downstream direction. Spatial changes in phytoplankton composition could also be illustrated and were especially apparent when the contribution of Cyanobacteria (blue-green bacteria or blue-green algae), Bacillariophyceae and Euglenophyceae to the total algal composition was taken into account.